1. Field of the Invention
The present invention relates to an optical forming process for forming three-dimensional objects.
2. Description of Related Art
A molding method has conventionally been known as the method for forming three-dimensional models from a resin. The molding method, however, requires a long period of time and significant costs to prepare a mold. In addition, a number of steps are required to complete a three-dimensional model, which make the process completed. To overcome these problems, an optical forming method which does not require a mold has been proposed (for example, Japanese Patent Applications Laid-open No. 35966/1987, No. 113925/1990, No. 64048/1994, and No. 125079/1995). This optical forming method comprises forming a cured resin layer (n) by selectively irradiating a non-cured layer (n) of a radiation- or photo-curable liquid resin according to a slice shape data (n) of a three-dimensional model, feeding the radiation- or photocurable liquid resin over this cured resin layer (n) to form a fresh, non-cured resin layer (n+1), selectively irradiating this non-cured layer (n+1) of a photo-curable liquid resin according to a slice shape data (n+1), and repeating this process, thereby obtaining a three-dimensional model consisting of laminated layers of the cured resin.
FIG. 2 is a drawing schematically showing a basic construction of an optical forming apparatus used for such a method. The apparatus comprises a fixed base (1), a container (2) in which a photo-curable resin (3) is contained, a light source apparatus (4) for irradiating the liquid surface of the photo-curable resin (3), and a stage (5) which supports the laminated object (6) of cured resin layers. The stage (5) is movable up and down. For example, when the stage (5) is moved downward from the position shown in FIG. 2, the photo-curable resin (3) is fed onto the surface (6A) of the laminated layer (6) to form a non-cured layer with a thickness corresponding to the descended distance of the stage (5).
One problem with this optical forming method is a considerably long period of time required for forming a great number of three-dimensional models with the same shape. Another problem is a limitation to the types of resin which can be applied to this method. This limitation leads to production of three-dimensional models which do not possess intended properties.
As a means to solve these problems, a forming method of a three-dimensional model has been proposed, which comprises fabricating a mold of a heat resistant resin by an optical forming method using a radiation-curable resin (such a mold is hereinafter called a "resin-made mold") and forming the target three-dimensional model using this resin-made mold from raw material resins which can exhibit intended characteristics. A great number of the three-dimensional models possessing the intended characteristics can be formed by this method without impairing the advantage of the simplicity in the optical forming process.
The addition of fillers to a photo-curable resin composition has been known from the patent applications JP-A-05286040, DE-A-4305201 and EP-A-393676 as an effective means for improving heat resistance and physical properties (reinforcing effects, . . . ) of a cured resin by an optical forming process. Inorganic fillers such as inorganic powder or inorganic fiber are given as examples of suitable fillers.
Because the fillers blended with a photo-curable resin composition usually have a density higher than that of the photo-curable resin, the fillers are easily sedimentated when the photo-curable resin composition is filled in the container. No optical forming objects (three-dimensional objects) with desired heat resistance and physical properties can be obtained from a photo-curable resin composition in which the fillers contained therein are dispersed only inadequately. When the state of dispersion of the fillers in the photo-curable resin composition in the container changes over time, particularly when the concentration of fillers decreases close to the liquid surface, the resulting optical forming object has different concentrations of the fillers in the lower part formed at the beginning of the forming and in the upper part which is formed close to the end of the optical forming operation. As a result, the properties of the resulting optical forming object is varied according to sections.
Further, the patent applications DE-A-4414775, DE-A-4417083 and JP-A-04118221 describe an optical forming apparatus for 3-dimensional models provided with a device to transfer an overflow of the resin composition back to the container in which the model is built.